A phenomenon known as activist gardening is taking off this spring in cities such as Portland, Detroit, Baltimore and Washington, where young urbanites are redefining the seemingly fusty pastime as a tool for social change. This is civil disobedience with a twist: Vegetable patches and sunflower gardens planted on decrepit medians and in derelict lots in an effort to beautify inner-city eyesores or grow healthful food in neighborhoods with limited access to fresh food.